Compressive receivers, sometimes referred to as microscan receivers, are broadband devices primarily operating in the microwave frequency range and are often employed to intercept any of various types of signals such as radar and communication signals in a dense signal environment.
Compressive receivers use a dispersive delay line (DDL) for compressing into a narrow pulse a frequency modulated (FM) signal which is generated from an input radio frequency (RF) signal. A fast sweeping local oscillator (SLO) is employed to convert the input signal into an FM signal. By measuring the positions of the output pulses in the time domain, the frequency of the input signal can be obtained.
The DDL possesses an inherent differential delay time and frequency separation or bandwidth. The time-bandwidth product of a DDL is the differential delay time multiplied by the bandwidth. The time-bandwidth product is an important characteristic which determines the processing gain of a compressive receiver. The differential delay time is related to the frequency resolution of the receiver, and the bandwidth of the DDL is related to the input bandwidth and the probability of intercept of the receiver.
The SLO possesses a frequency versus time slope which matches that of the DDL in amplitude but is opposite in sign. An RF input signal with constant frequency is converted to a linear FM signal at the output of a mixer in which the input signal is combined with the frequency sweep generated by the SLO. This signal is often modified by a weighting filter and is then compressed in the time domain into a pulse at the output of the DDL. A detector is employed to sense the amplitudes of the output pulses from the DDL, and determines the relative positions of the pulses in the time domain.
The width of the output pulses limits the use of wideband compressive receivers to those applications where high speed circuitry is available for processing the relatively narrow output pulses. In the past, in order to process the narrow output pulses, it has been necessary to employ video pulse stretching circuitry to increase pulse width or other high speed phased clocking circuitry to handle the short pulses.
The present invention is directed to eliminating the need for the special processing circuitry mentioned above by providing a system for increasing the width of the output pulses.